The present invention relates generally to deposits formed on surfaces in contact with hydrocarbon fluids, and more particularly, to a method of preventing or reducing the deposit of hydrocarbon fluid thermal degradation products on surfaces in contact therewith and to a metal article having a coated surface which inhibits the formation of gum and/or coke formed by thermal degradation of the fluid, without resorting to modification of the fluid, without adoption of special procedures and without installation of special equipment for their use.
As used herein, hydrocarbon fluid is defined as hydrocarbon liquids, hydrocarbon gases or mixtures thereof. As used herein, "hydrocarbon fluid degradation products" includes products which form from the hydrocarbons, for example, certain polymers resulting from thermal transformation of paraffins to cycloparaffins, aromatics and polycyclic molecules in the hydrocarbon, as well as products which result from actual decomposition of the fuel, e.g., carbon.
Because high temperature is usually associated with undesirable levels of hydrocarbon fluid deposit formation, the technical subject herein is customarily referred to as thermal instability, or in the case of fuels, as fuel instability. Flowing hydrocarbon fluids including lubricating oils, hydraulic oils and combustible fuels form gum and coke deposits on the surface of containment walls and other parts which they contact, when the fluid and/or surface are heated.
The mechanisms for formation of deposits from thermal instability have been studied and documented. In the case of fuels, it is generally accepted that there are two distinct mechanisms occurring at two levels of temperature. In the first mechanism, referred to as the coking process, as temperature increases from room temperature, starting at about 300.degree. F. (about 149.degree. C.) there is generally a consistent increase in the rate of formation of coke deposits up to about 1200.degree. F. (about 649.degree. C.) where high levels of hydrocarbon lead to coke formation and eventually limit the usefulness of the fuel. A second lower temperature mechanism starting at about room temperature, generally peaks at about 700.degree. F. (about 370.degree. C.) and involves the formation of gum deposits. This second mechanism is generally better understood than the coking process. It involves oxidation reactions which lead to polymerization which includes the formation of gums. Both coke and gum formation and deposits can occur simultaneously in the mid-temperature region.
Coke formation in hydrocarbons is discussed in U.S. Pat. No. 2,698,512, and heat stability of jet fuel and the consequences of thermal degradation of the fuel are discussed in U.S. Pat. No. 2,959,915, both patents being incorporated herein by reference in their entirety. These patents suggest specific formulations which place limitations on the fuel chemistry and impurities associated with hydrocarbon fuels so that the fuels will be usable at high temperatures without the typical formation of gums and coke.
Gum and coke formation are discussed in U.S. Pat. No. 3,173,247, which is incorporated by reference herein in its entirety. It is indicated therein that at very high flight speeds, heat must be transferred, particularly from the engine, to some part of the flight vehicle or to its load, and although the fuel which is stored on the vehicle, could serve to receive this heat, in practice, such procedure is unfeasible because jet fuels are not stable to the high temperatures which are developed at multi-Mach speeds, instead, they decompose to produce intolerable amounts of insoluble gum or other deposits, for example coke. As with the previously referenced patents, the solution to the problem has been directed toward limitations on fuel chemistry and impurities associated with the fuel.
The chemistry of the hydrocarbon fluid mixture and the chemistry of the containment vessel can have a major influence on deposit mechanisms and deposit rates at temperatures where it is most desirable to use the fluid. Hydrocarbon fluids contain impurities of which sulfur and dissolved oxygen from air, are major constituents. Gums are essentially vinyl polymers formed by reactions between oxygen and olefins in hydrocarbon fluids. Coke can also be in the form of carbon polymers and can have crystalline structures, and deposits formed from decomposition products of hydrocarbon fluids, are often observed to be a mixture of gum, coke, hydrocarbons and other impurities. Gums adhere to surfaces much in the same way as glues, and accordingly, they tend to entrap other solid particles such as coke, solid hydrocarbon impurities (or products), and the like and thereby form deposits on surfaces which they contact. In the lower temperature region where gum formation occurs, oxygen from air dissolved in the liquid is the major adverse ingredient. Boiling amplifies this adversity because of the oxygen concentration effect adjacent to hot walls. If oxygen is absent, gum formation is not likely to occur.
In much of the prior art, the problems associated with gum and coke thermal deposits has predominately dealt with bulk fluid chemistry and reactions which can take place within the fluid. These investigations have involved a wide range of hydrocarbon compositions and the presence of numerous impurities such as sulfur compounds, nitrogen compounds, oxygen and trace metals. It has been observed that deposits attached to containment walls often contain very large quantities of sulfur and nitrogen compounds or intermediates thereof in addition to gums and cokes. Little attention has, however, been given in the prior art to the role of the chemistry and reactions which take place in the vicinity the containment walls and the fluid.
In U.S. Pat. No. 3,157,990, certain phosphate additives are added to the monopropellant wherein the phosphates decompose in the reaction chamber and form a coating, probably a phosphate coating, on the internal generator surfaces, and it is suggested that this coating effectively inhibits carbon decomposition and scaling. In U.S. Pat. No. 3,236,046, which is incorporated by reference herein in its entirety, the interior surfaces of stainless steel gas generators are passivated with sulfurous materials to overcome deposition of coke on the surfaces of the gas generator, and passivation is defined as a pretreatment which substantially reduces initial catalytic coke formation.
In U.S. Pat. No. 4,078,604, which is incorporated by reference herein in its entirety, heat exchangers are characterized by thin-walled corrosion resistant layers of electrodeposited gold or similar corrosion-resistant metals on the walls of the cooling channels within the inner wall, and the cooling channels are covered with the electro-deposited layer of gold in order to make the surfaces corrosion resistant to such storable liquid fuels as fuming nitric acid. In this prior art case, the wall is protected from corrosion by the propellent, but the intent is not to prevent deposit formations.
Protective metal oxide films on metal or alloy substrate surfaces susceptible to coking, corrosion or catalytic activity are referred to in U.S. Pat. No. 4,297,150, which is incorporated by reference herein in its entirety, where it is first necessary to pre-oxidize a substrate surface and then to deposit on the pre-oxidized surface a metal oxide of calcium, magnesium, aluminum, gallium, titanium, zirconium, hafnium, tantalum, niobium or chromium by vapor phase decomposition of a volatile compound of the metal, wherein nitrogen, helium, argon, carbon dioxide, air or steam may be used as carrier gases for the metal compound, the volatile compound having at least one metal-oxygen bond.
In U.S. Pat. No. 4,343,658, reference is made to the protection of metal substrate surfaces against carbon accumulation when exposed to an environment wherein carbon-containing gases are decomposed by the use of tantalum and/or tungsten entities deposited and/or diffused into the surface of the substrate. According to U.S. Pat. No. 4,343,658, which is incorporated by reference herein in its entirety, filamentous carbon grows on surfaces at a reduced rate (by a factor of at least four) when the tantalum and/or tungsten entity deposited on the surface is decomposed at a temperature of 600.degree. C. to 1200.degree. C. to drive tungsten and/or tantalum metal into the substrate surface.
In Japanese patent application No. 57-12829, reference is made to preventing the adhesion of tar by spray coating a blend containing aluminum chloride and cobalt oxide on a surface to provide a coated surface which has a catalytic activity for the decomposition of tar compounds into compounds that can be vaporized at low temperatures. According to Japanese patent application No. 56-30514, when tar collects on a surface which has been spray coated with a blend of a tar decomposing catalyst chosen from titanium oxide, zirconium oxide, vanadium oxide, chromium oxide, molybdenum oxide, tungsten oxide, manganese oxide, iron oxide, cobalt oxide, nickel oxide, copper oxide, platinum, palladium, rhodium, ruthenium osmium or iridium and an inorganic binder of silicate, aluminum phosphate, glass, lithium, silicate solution, colloidal silica or alumina sol, it can be heated at 350.degree. C. for 60 minutes to remove the tar built up on the surface.
Thermal instability and fuel instability, referred to above, are becoming more significant with developing technology, and it will become even more significant as processes and machinery will be required to operate at higher temperatures as afforded by advances in materials technology and as the chemical quality of hydrocarbons for fuels, oils, lubricants, petrochemical processes (plastics and synthetics) and the like, decreases. Furthermore, hydrocarbon fluids, fuels and oils derived from non-petroleum sources, such as shale and coal, will have significantly more problems with thermal instability because of their high content of olefins, sulfur and other compounds. Accordingly, it is advantageous to provide coated articles and processes for preventing the formation of adverse degradation products and foulants in such applications where thermal instability, including fuel instability, is a problem as a result of exposure of such fluids to high temperatures.
In view of the foregoing, it can be seen that it would be desirable to provide coated metal articles, e.g., fuel containment articles for containing hot hydrocarbon fluid, in which or on which degradation products formed by thermal degradation of the hydrocarbon fluid is avoided, eliminated or reduced. It would also be desirable to provide a method of protecting metal surfaces which contact hot hydrocarbon fluid, from the deposit of degradation products of the hydrocarbon fluid. It can also be seen from the foregoing that it is desirable to provide methods and articles for use with hydrocarbon fuels wherein the hydrocarbon fuel can be used as a heat sink without the undesirable deposit of insoluble gums, coke, sulfur compounds or mixtures thereof on surfaces, e.g., containment surfaces. It is also desirable to provide methods and articles for containment of vaporized fuel to reduce NO.sub.x emission and to provide methods and articles for containment of low quality fuels derived from coal, shale and low grade crude oil.
The disadvantages of the prior art processes and techniques discussed above involve the need to alter the hydrocarbon chemistry, maintain strict control of impurities and/or provide additives and special processing such as pre-oxidizing treatment, passivation treatments and/or post-decomposition heat treatments using excessive amounts of heat, and the like. All of these techniques constrain the use of the fluid, increase cost and promote uncertainty as to the quality level of the fuel or treatment at a particular time. Furthermore, there are a multitude of processes, systems and devices including petrochemical processes, machine tools, automobile engines, aircraft gas turbine engines, and marine and industrial engines in which surface deposits from hydrocarbon fluids, fuels and oils are a major problem. Deposits can foul heat exchangers, plug fuel injectors and lubrication distribution jets, jam control valves and cause problems with many other types of operating and control devices associated with hydrocarbon fluids, fuels and oils. It is a primary objective of this invention to overcome these disadvantages.